OBJECTIVE: To examine the endometrial effects of three different doses of progesterone administered vaginally. METHODS: Forty women 25-41 years old deprived of ovarian function received estradiol (E2) for 28 days. From days 15 to 27, a new mucus-like vaginal gel of progesterone was administered every other day, randomly, dosed at 45 mg (group A, n = 14), 90 mg (group B, n = 13), or 180 mg (group C, n = 13). Plasma gonadotropins, estrone, E2, and progesterone were measured. An endometrial biopsy was performed on day 20 (n = 20) or 24 (n = 20) for endometrial dating and for estrogen and progesterone receptor determinations. RESULTS: Plasma estrogen levels were in the menstrual cycle range. Mean progesterone levels were lower in group A (2.4 +/- 0.2 ng/mL) than in group B (3.6 +/- 0.2 ng/mL) or C (3.4 +/- 0.4 ng/mL) (P